feat: billing — usage tracking per SQS operation in PostgreSQL

- New package app/billing: Init/RecordUsage/Close with auto-migrate
- Integration in actionHandler: record tenant_id, operation, msg_count, msg_bytes
- Helm chart: billing section in values.yaml, secret-billing.yaml, env vars in deployment
- Optional: billing disabled by default (BILLING_PG_HOST not set = no-op)
- Table: sqs_usage_records with index on (tenant_id, recorded_at)

go.mod

@@ -7,7 +7,9 @@ require (
 	github.com/google/uuid v1.6.0
 	github.com/gorilla/mux v1.8.0
 	github.com/gorilla/schema v1.4.1
+	github.com/lib/pq v1.12.3
 	github.com/mitchellh/copystructure v1.2.0
+	github.com/redis/go-redis/v9 v9.18.0
 	github.com/sirupsen/logrus v1.9.0
 	github.com/stretchr/testify v1.7.0
 )
@@ -19,7 +21,6 @@ require (
 	github.com/kr/pretty v0.1.0 // indirect
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/redis/go-redis/v9 v9.18.0 // indirect
 	go.uber.org/atomic v1.11.0 // indirect
 	golang.org/x/sys v0.13.0 // indirect
 	gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 // indirect